Erythritol, a sugar alcohol, is 60-80% as sweet as sucrose. Yet, it has a calorific value only about one tenths that of sucrose and does not contribute to dental caries. Also, unlike many sugar alcohols, it does not cause diarrhea. Further, erythritol possesses excellent processing properties: It is heat-stable; and it does not react with amino groups and therefore does not cause browning of organic substance.
Erythritol can be found in lichen, hemp leaves, mushrooms, fermentative foods (e.g., wine and soy sauce), and microorganisms. Among erythritol-producing microorganisms are yeast strains of the Pichia, Candida, Torulopsis, Trigonopsis, Moniliella, Aureobasidium, and Trichosporon genera.
The present invention relates to new yeast strains which are capable of converting glucose into erythritol in a simple medium.
The yeast strains of this invention are characterized by an absence of motile spores or zoospore (i.e., spores having flagella); septate mycelia (i.e., mycelia having dividing walls); asexual reproduction (i.e., reproduction not involving kasogamy and meiosis); an absence of reniform cells; conidia optionally formed on short denticles (i.e., small tooth-like projections) but not on elongate stalks; an absence of ballistoconidia (i.e., forcibly discharged conidia); non-monopolar budding on a broad base (i.e., a multiplication process in which there is no development of a new cell from a single-pole outgrowth); acropetal chains of blastoconidia (i.e., conidia characterized by a marked enlargement of a recognizable conidial initial before the initial is delimited by a septum); dark brown, thick-walled chlamydospores (i.e., asexual 1-celled spores each originating endogenously and singly within part of a pre-existing cell by the contraction of the protoplast); a fermentative ability (i.e., an ability to ferment semi-anaerobically at least one carbon source); an ability to assimilate sucrose, glycerol and maltose; an inability to assimilate lactose; an inability to ferment galactose; an ability to grow in a vitamin-free medium; and an ability to grow at 25xc2x0 C., 30xc2x0 C., 35xc2x0 C. and 36xc2x0 C. Optionally, the yeast strains of this invention can be further characterized by an ability to ferment sucrose, glucose (i.e., D-glucose) or maltose; or an ability or inability to assimilate galactose.
The strains"" ability or inability to ferment or assimilate a specific carbon source, to grow in a vitamin-free medium, and to grow at a specific temperature can be determined following the procedures described in the actual examples below.
Also contemplated within the scope of this invention are yeast strains characterized by the morphological traits set forth above and the physiological traits listed in Table 1, 2, 3, 4, 5 or 6 below. Examples include, but are not limited to, 6 strains deposited on Jan. 27, 2000 at the American Type Culture Collection, 10801 University Blvd., Manassas, Va. 20110-2209, USA. The accession numbers of the deposited strains are PTA-1227, PTA-1228, PTA-1229, PTA-1230, PTA-1231, and PTA-1232. Mutants derived from the deposited strains are also within the scope of this invention.
The yeast strains of the present invention are closest to Moniliella acetobuten. Indeed, their morphological traits as described above coincide with those of M. acetobuten. On the other hand, their physiological characteristics differ only slightly from M. acetobuten. For example, unlike M. acetobuten, the strains of this invention do not assimilate lactose. Further, they are capable of converting glucose to erythritol in a simple medium (e.g., glucose and yeast extract only) at a rate unexpectedly higher than M. acetobuten. 
Other features or advantages of the present invention will be apparent from the following detailed description (including actual examples) and also from the appending claims.
Yeast strains of the present invention can be isolated from natural sources, e.g., samples having high sugar contents such as honey, preserved fruit, and pollen. Each strain is identified based on its capability to convert glucose to erythritol and its various morphological and physiological traits. A yeast strain of this invention is capable of converting 1 g of glucose into at least 0.3 g of erythritol (i.e., conversion rate xe2x89xa730%). The conversion rate is determined by culturing the strain in a 10-ml broth containing 30% glucose and 1% yeast extract (initial cell density 1xc3x97105 cells/ml) in a 50 ml flask in a rotary shaker at 150 rpm and 30xc2x0 C. for 6 days. The morphological traits are determined following growth on 4% malt extract/0.5% yeast extract agar for 10 days at 20xc2x0 C. See, The Yeasts, A Taxonomic Study, Edited by Kurtzman et al., 4th Ed., page 785, Elsevier, Amsterdam (1998). The physiological traits, on the other hand, are determined by the methods described in the actual examples below.
Other yeast strains of this invention can be variants derived from strains isolated from natural sources. For example, such strains may be mutants obtained by UV irradiation, N-methyl-Nxe2x80x2-nitrosoguanidine treatment, ethyl methanesulfonate treatment, nitrous acid treatment, acridine treatment, and the like. They also may be recombinant strains genetically produced by means of cell fusion or recombinant DNA techniques.
A skilled person in the art can obtain and utilize yeast strains of the present invention to the fullest extent based on the following specific examples, which are merely illustrative and not limitative of the remainder of the disclosure. All publications cited herein are incorporated by reference.